Configuring the Ethernet Switches

This chapter gives an overview of configuration tasks for the 4-port Fast Ethernet (FE) switch and for the Gigabit Ethernet (GE) switch that services the embedded wireless access point on the Cisco 819 Integrated Services Routers (ISRs).

The FE switches are 10/100Base T Layer 2 Fast Ethernet switches. Traffic between different VLANs on a switch is routed through the router platform with the switched virtual interface (SVI).

The GE switch is a 1000Base T Layer 2 Gigabit Ethernet switch with an internal interface between the router and its embedded wireless access point.

Any switch port may be configured as a trunking port to connect to other Cisco Ethernet switches.

CDP runs on all LAN and WAN media that support Subnetwork Access Protocol (SNAP). Each CDP-configured device sends periodic messages to a multicast address. Each device advertises at least one address at which it can receive SNMP messages. The advertisements also contain the time-to-live, or hold-time information, which indicates the length of time a receiving device should hold CDP information before discarding it.

IGMP Snooping

IGMPv3 provides support for source filtering, which enables a multicast receiver host to signal to a router which groups the receiver host wants to receive multicast traffic from and from which sources this traffic is expected. Enabling the IGMPv3 feature with IGMP snooping on Cisco ISRs provides Basic IGMPv3 Snooping Support (BISS). BISS provides constrained flooding of multicast traffic in the presence of IGMPv3 hosts. This support constrains traffic to approximately the same set of ports as IGMPv2 snooping does with IGMPv2 hosts. The constrained flooding only considers the destination multicast address.

Storm Control

Fallback Bridging

Overview of SNMP MIBs

Simple Management Network Protocol (SNMP) development and use is centered around the Management Information Base (MIB). An SNMP MIB is an abstract data base and it is a conceptual specification for information that a management application may read and modify in a certain form. This does not imply that the information is kept in the managed system in that same form. The SNMP agent translates between the internal data structures and formats of the managed system and the external data structures and formats defined for the MIB.

The SNMP MIB is conceptually a tree structure with conceptual tables. Cisco Layer 2 Switching Interface MIB is discussed in more detail in the next section. Relative to this tree structure, the term MIB is used in two senses. In one sense, it is actually a MIB branch, usually containing information for a single aspect of technology, such as a transmission medium or a routing protocol. A MIB used in this sense is more accurately called a MIB module and is usually defined in a single document. In the other sense, a MIB is a collection of such branches. Such a collection might comprise, for example, all the MIB modules implemented by a given agent or the entire collection of MIB modules defined for SNMP.

A MIB is a tree where the leaves are individual items of data called objects. An object may be, for example, a counter or a protocol status. MIB objects are also sometimes called variables.

BRIDGE-MIB for Layer 2 Ethernet Switching

The Layer 2 Ethernet Switching Interface BRIDGE-MIB is supported in the Cisco 819 platforms. The BRIDGE-MIB enables the user to know the Media Access Control (MAC) addresses and spanning tree information of the Ethernet switch modules. The user can query the MIB agent using the SNMP protocol and get the details of Ethernet switch modules such as MAC addresses of each interfaces and spanning protocol information.

The Bridge-MIB uses the following approaches to get the L2 layers BRIDGE-MIB information:

Community-string-based approach

Context-based approach

In the community-string-based approach, one community string is created for each VLAN. Based on the query, the respective VLAN MIB is displayed.

To get the BRIDGE-MIB details, use the snmp-server community public RW command in the configuration mode.

Router(config)#snmp-server community public RW

Use the following syntax to query the SNMP BRIDGE-MIB details:

snmpwalk -v2c <ip address of the ISR, …> public .1.3.6.1.2.1.17

snmpwalk -v2c <ip address of the ISR, …> public@2 .1.3.6.1.2.1.17

snmpwalk -v2c <ip address of the ISR, …> public@3 .1.3.6.1.2.1.17

Note When you create a VLAN “x”, the logical entity public@x is added. If you query with the public community, the L3 MIB is displayed. When you query with public@x, the L2 MIB for VLAN “x” is displayed.

In the context-based approach, the SNMP context mapping commands are used to display the values for L2 interfaces. Each VLAN is mapped to a context. When the user queries with a context, the MIB displays the data for that specific VLAN, which is mapped to the context. In this approach, each VLAN is manually mapped to a context.

To get the BRIDGE-MIB details, use the following commands in the configuration mode:

Router(config)#Routersnmp-server group public v2c context bridge-group

MAC Address Notification

MAC address notification enables you to track users on a network by storing the MAC address activity on the switch. Whenever the switch learns or removes a MAC address, an SNMP notification can be generated and sent to the NMS. If you have many users coming and going from the network, you can set a trap interval time to bundle the notification traps and reduce network traffic. The MAC notification history table stores the MAC address activity for each hardware port for which the trap is enabled. MAC address notifications are generated for dynamic and secure MAC addresses; events are not generated for self addresses, multicast addresses, or other static addresses.

Configuring VLANs

VLANs on the FE Ports

Perform these steps to configure VLANs, beginning in configuration mode.

Command

Purpose

Step 1

interface
fe
port

Selects the Fast Ethernet port to configure.

Step 2

shutdown

(Optional) Shuts down the interface to prevent traffic flow until configuration is complete.

Step 3

switchport

Configures the Fast Ethernet port for Layer 2 switching.

Note You must enter the switchport command once without any keywords to configure the Fast Ethernet port as a Layer 2 port before you can enter additional switchport commands with keywords. This command creats a Cisco default VLAN.

This configuration sets the default trunking administrative mode to
switchport mode dynamic desirable
and the trunk encapsulation to
negotiate
.

By default, all VLANs created are included in the default trunk.

Step 4

switchport
access vlan
vlan_id

Creates instances of additional VLANs. Allowable values of
vlan_id
are 2 to 4094, except for reserved values of 1002 to 1005.

VLANs on the GE Port

Because the GE port is an internal interface that services only the router’s embedded access point, it cannot be configured only with the command
switchport access vlan X
, where X is other than 1. It may, however, be configured in trunk mode. This may be done by performing the following steps, beginning in configuration mode.

Comand

Purpose

Step 1

interface
Wlan-GigabitEthernet0

Selects the Gigabit Ethernet port to configure.

Step 2

switchport mode trunk

Places the port in trunk mode.

Step 3

switchport access vlan
vlan_id

(Optional) Once the port is in trunk mode, it may be assigned a VLAN number other than 1.

Configuring MAC Table Manipulation

The topic of enabling known MAC address traffic deals with port security. Port security can be either static or dynamic.

Static port security allows the user to specify which devices are allowed access through a given switch port. The specification is done manually by placing allowed device MAC addresses in the MAC address table. Static port security is also known as MAC address filtering.

Dynamic port security is similar. However, instead of specifying the MAC address of the devices, the user specifies the maximum number of devices that will be allowed on the port. If the maximum number specified is more than the number of MAC addresses specified manually, the switch will learn the MAC address automatically, up to the maximum specified. If the maximum number specified is less than the number of MAC addresess already specified statically, an error message will be produced.

The following command is used to specify static or dynamic port security.

In support of the IGMPv3 feature in Cisco IOS Release 12.4(15)T, the
groups
and
count
keywords were added to the
show ip igmp snooping
command, and the output of the
show ip igmp snooping
command was modified to include global information about IGMP snooping groups. Use the
show ip igmp snooping
command with the
groups
keyword to display the multicast table learned by IGMP snooping for all VLANs or the
show ip igmp snooping
command with the
groups
keyword,
vlan-id
keyword, and
vlan-id
argument to display the multicast table learned by IGMP snooping for a specific VLAN. Use the
show ip igmp snooping
command with the
groups
and
count
keywords to display the number of multicast groups learned by IGMP snooping.